Characterisation of the bound lipids at the wool fibre surface by time of flight secondary ion mass spectrometry (ToF-SIMS)

Abstract

Negative ion time of flight secondary ion mass spectrometry (ToF-SIMS) analysis of rigorously cleaned untreated, hydroxylamine and potassium hydroxide (KOH)-treated wool fabrics, produced with minimal finishing, has revealed the possible presence of hitherto unidentified bound acids at the surface of the wool fibre. Spectral peaks have been assigned to C8 through to C22 fatty acids, the odd numbered acids of which have been assigned to anteiso acids, (a), akin to the anteiso acid 18-methyleicosanoic acid known to be the dominant fatty acid bound to keratin fibre surfaces. This has included the assignment of some small peaks to hydroxylated aC18H36(OH)COS− and aC20H40(OH)COS− species, where the hydroxyl group is attached to the asymmetric 16 or 18 numbered carbon atom in the alkyl chain, respectively. The study has confirmed that many of the acids are bound to the fibre surface by thioester linkages (about 70%) and that the remainder is bound by oxygen ester acyl linkages. ToF-SIMS analysis of hydroxylamine-treated wool has confirmed the removal of all the thioester bound surface lipids, as well as lesser quantities but not all of the acyl bound lipids. In contrast, a 2 h KOH treatment removes all surface accessible bound lipids. Positive ion ToF-SIMS spectral analyses have not proved useful in characterising the possibly thiol rich fibre surface remaining after hydroxylamine treatment. The formation of characteristic immonium ions from surface cysteine residues after hydroxylamine treatment has been shown not to occur for free cysteine and so is unlikely to occur for such residues at the wool fibre surface. Scanning electron microscopy and transmission electron microscopy analyses have also been performed to assess the extent of the hydroxylamine reaction with the fibre. The implications of these findings for currently proposed models of the wool fibre surface, and for exploiting reactions of hydroxylamine followed by other electrophilic reagents to produce modified wool surfaces, are also discussed.